1) Field of the Invention
The present invention relates to automotive test equipment such as a machine for balancing motor vehicle wheels of different sizes and types, and in particular to a system for inputting wheel information concerning the particular wheel under test so that the wheel balancer may determine the required imbalance correction weight and point of placement on the wheel/tire assembly.
2) Discussion of Prior Art
It is well known in the wheel balancing art that certain parameters of the tire/wheel combination must be fed into the machine before it can determine the magnitude and angle of imbalance for a given correction weight plane (usually the lip of the wheel). These inputs typically involve rim diameter, rim width, and some reference distance of the rim to the balancer.
Although automatic data entry systems exist for this function (see U.S. Pat. No. 4,341,119 to Jackson et al, U.S. Pat. No. 4,576,044 to Boni, and U.S. Pat. No. 3,741,016 to Hofmann), all known wheel balancing machines have a manual method for feeding in these parameters (either standing alone or in addition to the automatic system). Such a manual system is required in the event of automatic data entry system failure. Moreover, lower priced balancers are sold with only manual input systems because of the expense of automatic systems.
The manual systems on known machines include either potentiometers (pots), keypad(s), or a simple up/down membrane switch for parameter entry. Each type of manual entry system has its own advantages.
The exclusive use of pots (illustrated by wheel balancers sold by Hunter Engineering Company under the trade designations S700 and S800) offers the user instant visual access to the dialed-in parameter without the need for an electronic display for that parameter. In these machines, the range of possible values is simply provided by an artwork scale on the front panel of the balancer. Dialing a pot is faster than punching a series of keys for each value. In addition, hands dirtied from handling tires soil the knob, not the artwork (as with a keypad).
Since the knob of the potentiometer is a strictly mechanical device, static discharge from the user to the knob does not damage the balancer electronics. Contrast that with keypads. The keypad is a natural path for human static discharge damage to electronics, thus requiring additional printed circuit (pc) board real estate and the attendant costs of protective devices.
Finally, the feel of a knob is desirable over a keypad or a simple up/down membrane switch, especially over inexpensive membrane-type keypads found on wheel balancers. These keys must endure constant use, and reliability is questionable for this application. (For comparison, rotary knobs are the norm on high-end audio equipment of any perceived quality.)
On the other hand, the exclusive use of keypad(s) (illustrated by U.S. Pat. No. 4,502,328 to Wood et al.) offers the user unambiguous data input because no analog to digital conversion takes place (as does occur with the use of pots). The user does not have to interpolate between two marks on panel artwork when confronted with non-standard values such as 5.6 inches for wheel width--a keypad can enter 5.6 exactly. A keypad does not experience humidity, age drift, component tolerance, or linearity problems as can pots.
Pots also require the user to physically move the dial out of the range of panel artwork scale values, typically to an "external" position, when it is desired to input parameters via an automatic entry device, such as those described above. If this is not done, the dial could erroneously be pointing to a front panel artwork value while the machine would actually be using a different value obtained from the automatic system. Keypads eliminate this problem since there is no artwork scale involved.
The keypad offers the assembly advantage of "plug in and go," whereas a pot requires careful alignment to panel artwork to work in its proper rotary range. (The pot also must never move from its originally installed position.) Finally, the keypad input value is displayed electronically (usually on an LED display), which is easier to read than a pointer of a pot knob against an artwork scale printed on a panel (especially at any distance over two feet).